The present invention relates to the production of polyolefins. The invention relates in particular to a process according to the preamble of claim 1 for the production of polyolefins in a reactor system which comprises at least one gas-phase reactor wherein at least one monomer is polymerized. According to the process, hydrogen and possibly other gaseous or vaporized light components are separated from the fluid being directed to the gas-phase reactor.
The invention is applicable to the separation of light components from the stream between two polymerization reactors and to the removal of volatile (uncondensable) components from the circulation gas of a gas-phase reactor used for the preparation of polyolefins.
Olefins are polymerized into polyolefins in a gas-phase reactor in the presence of, for example, Ziegler-Natta type catalysts or metallocene catalysts. The forming polymer mass, which contains an active catalyst, is fluidized in :a fluidized bed reactor by using a hydrocarbon stream. This stream contains a monomer (typically ethylene or propylene) and possibly a comonomer (C2-C10 olefins or diolefins, preferably ethylene, propylene, 1-butene or hexene), as well as hydrogen, which controls the molecular weight. The hydrocarbon stream may also contain high concentrations of a substance inert in the reaction (typically nitrogen or propane).
During the polymerization there is withdrawn from the gas-phase reactor a gas stream which contains at least some amount of unreacted monomer and which is returned to the gas-phase reactor as circulation gas. Most of the heat of reaction is removed by cooling this circulation gas. In order to ensure sufficient cooling efficiency and to achieve suitable fluidization velocities, the circulation gas stream is typically quite large. From the viewpoint of the functioning of the condenser it is preferable to remove from the stream the components with the lowest molecular weights, in particular hydrogen.
Nowadays hydrogen is removed from the gas-phase reactor either from the circulation gas stream or from the recovery stream together with the purge stream.
There are considerable disadvantages associated with prior art solutions. Thus the most typical method of lowering the concentration of hydrogen in the reactor is to increase the off-gas stream of accumulating inerts to a level close to the maximum capacity of the gas removal system. The gas removed is directed, for example, to the flare for burning or as feed to cracking. These methods are bad, expensive and slow, since both chemicals and production capacity are lost, which is due to the fact that a product of deficient quality is produced.
A further disadvantage of the prior art is that, when a typically relatively low concentration of hydrogen is lowered, the gases leaving together with the hydrogen cause a large financial loss to the polyolefin plant.
The accumulation of hydrogen and other light components in the gas streams of the polymerization process constitutes a problem also when polyolefins are prepared in a plant where two reactors are coupled together. Owing to the light components, such as hydrogen and inert compounds, present in the stream between the reactors, all products cannot be prepared by directing the mixture emerging from one reactor directly to the following gas-phase reactor. The reactor product is therefore directed to a product separator, where its pressure is lowered, whereupon the liquid possibly arriving together with the product vaporizes at least in part, or the gas expands and is recyclable to the first reactor. The pressure of the recycled stream is raised by using a compressor. Owing to the recycling, inert components tend to accumulate in the reactor.
In both of the cases mentioned above, particularly significant among the light components are hydrogen and lower alkanes, such as methane, ethane and propane, as well as nitrogen and other inert gases. Hydrogen is used for controlling the molar mass of the polymer, and the amount used varies according to the grade of the polymer. Reducing the proportion of hydrogen in the gas-phase reactor feed is important in situations in which it is desired to prepare long-chain polymers in a gas phase.
The object of the present invention is to eliminate the disadvantages of the prior art and to provide a novel process for the preparation of polyolefins in a reactor system having at least one gas-phase reactor.
The invention is based on the basic idea that hydrogen and possible other light components are separated by membrane separation from the fluid (liquid or gas stream) being fed into the gas-phase reactor, which fluid can comprise the circulation gas stream of the reactor or the effluent of the preceding reactor.
The use of membrane systems for the separation of hydrogen from process streams of polyolefin reactors is known per se from JP published application 08/151.413. In the said prior known process, gas to be separated under a high pressure from a gas-phase reactor is treated in a high-pressure membrane system to remove hydrogen, whereafter the gas stream is returned to the reactor.
The known technical solution has the disadvantage of a low degree of separation. Since the gas is separated under a high pressure, the polymer product will, even after the first separation step, contain volatile compounds, such as polymerization diluent and unreacted monomer, which have to be removed from the polymer separately. Also, no mention of the use of the process in connection with a grade change in the gas-phase reactor can be found in the publication, and evidently it is not applicable for this purpose, since in a separation carried out under a high pressure, relatively large amounts of monomer are left in the product polymer.
There are also other previously known membrane solutions. Thus, U.S. Pat. No. 4,740,550 describes a process for the preparation of a propylene-ethylene copolymer so that the propylene is first polymerized in two reactors and is then fed together with ethylene into a third reactor. According to the publication, hydrogen is removed from the circulation of the third reactor (i.e. the last gas-phase reactor) by using a hydrogen-selective hydrogen-removing membrane. In this known solution, the gas coming from the reactor and containing, for example, hydrogen, ethylene and propylene, is directed to a gas scrubber, to which ethylene and propylene which have condensed in the scrubber and have thereafter been cooled are recycled. The separation of the hydrogen-containing gas from the condensed components is thus based on the adding of cold liquid into the scrubber.
The present invention differs from the prior art in that the fluid to be fed into the gas-phase reactor is subjected, before being fed into the reactor, to a phase change in order to separate uncondensable compounds from liquid and possibly solid compounds. After at least a partial vaporization or condensation of the fluid, the gas made up of uncondensed and possibly vaporized compounds is recovered and; directed to membrane separation.
When it is desired to treat the stream between two reactors by using the option according to the invention, in accordance with prior art the product of the first reactor is directed to separation cyclones or corresponding apparatus for separating the reaction medium. According to the present invention, instead of vaporizing any possible liquid and directing it to a compressor, as in the prior art, a portion of the possible liquid is vaporized or the gas is allowed to expand in one or more steps, and the obtained gases are directed through a membrane system, whereby most of the hydrogen is removed. The hydrocarbons separated from the hydrogen are returned into the exit stream from the first reactor, and the said stream (fluid) is then fed into the gas-phase reactor. Thus it is possible to prepare from the effluent of the first reactor an intermediate product which can as such be fed into the second reactor (gas-phase reactor).
In the treatment of the circulation gas of the gas-phase reactor, there is withdrawn from the reactor a first gas stream, which is cooled to a first condensing temperature to condense it at least in part and to separate the liquid from the gas. Including the first gas stream, uncondensed compounds are separated in a membrane separator unit from the gas obtained from its condensation. The components obtained from the membrane separation are treated separately, and they are returned to the gas-phase reactor or are directed to distillation or a similar further treatment.
More specifically, the present invention is mainly characterized in what is stated in the characterizing part of Claim 1.
Considerable advantages are gained by means of the present invention. Thus, the production capacity of the gas-phase reactor will increase by at least 20%, i.e. to correspond to the capacity of a reactor system equipped with a cooling circulation operating with partial condensation. Control of the hydrogen concentration is sensible and economical. When the circulation gas compressor stops, the gas-phase reactor can be emptied rapidly in order to prevent the formation of agglomerates, since components can be removed from the hydrogen control cycle in a liquid phase. The off-gas is easily distilled. since it contains very little hydrogen. If the gas-phase reactor feed is in a liquid phase, there is saved in the polymer feed system (including pneumatic conveying) the flash tank, the removal of dust from the gas and the recovery compressor, and the distillation columns are reduced. If partial vaporization is installed in their place, even then the required gas treatment system will be substantially smaller and more economical than in the prior art. This is the case because hydrogen can be removed effectively from the gas-phase circulation. The invention is especially advantageously applicable to situations in which the diluent used in the gas-phase reactor is a heavy hydrocarbon such as butane, and/or a heavy comonomer such as 1-butene or 1-octene.
The condensation can be made more affective through the removal of light components, in particular hydrogen, from the circulation stream of the gas-phase reactor.
By means of the present invention, the change of grade of the polyolefin prepared in the reactor can be sped up. Although membranes usually do not make possible as precise a separation as, for example, distillation, a sufficiently high degree of separation in terms of polymer grade change can be achieved by the solution according to the invention. At the same time the costs of investment in equipment can be maintained low. By means of the invention, at least approx. 50%, preferably approx. 80-90%, of the hydrogen present in the gas stream subjected to treatment is removed.
In connection with the present invention it has been observed that a result especially advantageous in tenns of grade change is achieved by feeding into the membrane separator system a gas stream the composition of which substantially corresponds to the composition in the gas-phase reactor as regards small-molecular components. By xe2x80x9csmall-molecularxe2x80x9d components is meant hydrogen and/or nitrogen, as well as monomers and/or diluent residues, which in a gas-phase reactor are adsorbed to polymer particles. Such a gas stream is obtained either by withdrawing a gas stream from the top of the gas-phase reactor or by lowering the pressure of the reactor effluent and by separating the gas phase from the solid phase. The proportion of small-molecular components remains substantially unchanged.
In the above-mentioned JP published application, the system is placed at a point after a product separator operating under high pressure, in which case it has been necessary to provide a separate cycle for the system. In the present case, the membrane system is combined with a cycle necessary even otherwise; preferably the membrane system is used for separating hydrogen from the circulation-gas cooling stream or the recovery stream of the gas-phase reactor. This option improves the operation of the condenser in the circulation gas stream, if the membrane system is fitted in the circulation-gas cooling stream. If the membrane system is placed in the recovery stream, a compressor even otherwise needed is utilized, and thus a good operating environment is created for the membrane system. By both methods the speed of polymer grade change is increased, and thereby the profitability of the plant is improved. By means of the invention the costs incurred from a separate cycle constructed for the membrane system are avoided.
In an operation according to the JP publication, the composition of the gas introduced into membrane separation differs substantially from the small-molecule composition in the gas-phase reactor, since, when the product separation takes place substantially under the reactor pressure, heavy components of the reaction mixture leave the reaction gas in a larger proportion by volume. In a preferred embodiment according to the present invention, product separation takes place under a pressure: substantially lower than the reactor pressure. Thereby the reaction gas leaving together with the product polymer is decreased, and the change in its composition is very slight, since the comonomers and heavy diluent components are substantially entirely vaporized, and only the possibly oligomeric byproducts of the reaction remain in part unvaporized. The mutual proportions of the light, 20 i.e. small-molecular components, such as hydrogen, lower alkanes and alkenes, in particular methane, ethane and ethene, are substantially the same as in the stream obtained from the reactor.
The purity of the product is additionally maximized through the placement of the membrane system at a point after the low-pressure product separator. At the same time the reactive components are exploited maximally by being returned to the polymerization reactor (i.e. gas-phase reactor), where a product is prepared therefrom. The membrane separates from the stream the lightest components and thereby prevents them from accumulating in the circulation.
In the present invention it is therefore possible to operate so that the reactor pressure is lowered (if hydrogen is separated from the polymer product and the membrane system is located in the recovery stream).
According to another preferred embodiment, the separation is carried out from the circulation gas of the reactor. In this respect the invention to some degree resembles the process of U.S. Pat. No. 4,740,550; however, the composition of the gas directed to the membrane system is completely different, since in the present invention the condensed lower hydrocarbon compounds are not used for scrubbing the gas.
Considerable advantages are achieved when the present invention is applied to a process made up of at least one loop reactor and a gas phase reactor. Thus, a process implemented according to the invention is economically better than current systems, since the cooling need of the second reactor is smaller. The second reactor is easier to control, and grade changes are more rapid and easier, whereupon less product of a deficient quality is produced. The current compressor between the two successive reactors can be replaced with a considerably smaller one.
The removal of hydrogen at a point between the reactors is especially advantageous in a case in which the latter reactor is operated in such a manner that from the top of the reactor there is withdrawn a gas stream which, after partial or total condensation, is recycled to reactor feed.
The invention will be examined below in greater detail with the help of a detailed description, the accompanying drawings and a few embodiment examples.